In this thesis, the effect of various thermomechanical controlled processing parameters on the transformed microstructural evolution and grain refinement of an API X-80 high temperature processing steel was investigated by conducting plane strain compression tests followed by continuous cooling. Increasing the strain magnitude below the recrystallisation-stop temperature (T5%) from 0 to 0.5 with a cooling rate of 20˚C/s, a transition from bainitic ferrite (BF) to acicular ferrite (AF) occurs and grain refinement, microhardness and martensite/retained austenite (M/A) constituent refinement is improved. Once the magnitude of strain2 is increased to 0.7, dynamic recrystallisation (DRX) and subsequent static recrystallisation/meta-dynamic recrystallisation (SRX/MDRX) were triggered, weakening the beneficial influences of the austenite deformation. Applying continuous cooling with various cooling rates, ranging from 0.5˚C/s to 50˚C/s, to both fully recrystallised and fully unrecrystallised austenite, the effect of continuous cooling rate was investigated. Based on the results of the effects of austenite deformation and continuous cooling rate on AF transformation, the introduction of intragranular nucleation sites and halting of BF laths nucleated on austenite grain boundaries are found as two conditions that should be fulfilled for the occurrence of acicular ferrite transformation in pipeline steels. The effect of prior-austenite grain size (PAGS) on the microstructural evolution from both recrystallised and unrecrystallised austenite was studied. For microstructures transformed from recrystallised austenite, increasing the PAGS from 22.3 μm to 62.8 μm, the morphology of the transformed BF microstructure is changed and the effective grain size is reduced. For microstructures transformed from unrecrystallised austenite, reducing the PAGS from 62.8 μm to 37.0 μm, the volume fraction of AF is increased and the effective grain size is reduced from 4.5 μm to 2.9 μm. However, further reducing the PAGS from 37.0 μm to 22.3 μm, there are not significant changes on the transformed microstructures and the grain refinement.